Changes

Gas is exchanged in the lungs by diffusion— the process of molecules moving from an area of high concentration to an area of low concentration. Upon inhalation, oxygen molecules are directed into the alveoli by the bronchioles. The oxygen then diffuses into the blood from the alveoli, and the CO₂ molecules from the blood (a waste product of cellular respiration) enter the alveoli and are then exhaled. In order for this diffusion to occur, there must be a higher concentration of oxygen molecules in the alveolar air than in the blood, and a higher CO₂ concentration in the blood than in the alveoli. This is achieved through constant respiration, bringing in air that is rich in oxygen and scant in CO₂.

*[http://people.eku.edu/ritchisong/301notes6.htm Eastern Kentucky University, BIO 301]